1. Field of the Invention
The invention relates to a method for making a foam sheet, more particularly to a method for making a foam sheet having ventilation holes.
2. Description of the Related Art
A foam sheet made of polyurethane is formed with a plurality of holes spontaneously after foaming. Therefore, the insole made therefrom has good ventilation. However, polyurethane is a relatively expensive material for the insole. Another kind of foam sheet for the insole is made of ethylene-vinyl acetate copolymer (EVA), which is relatively inexpensive as compared to polyurethane. However, the foam sheet made of EVA is not formed with ventilation holes after foaming. Therefore, additional processing is required to provide the ventilation holes for the foam sheet made of EVA. As shown in FIG. 1, a conventional method for making a foam sheet having ventilation holes is shown to include the steps of preparing a mold having an upper mold part 1 and a lower mold part 2 provided with a mold cavity 201; closing the upper and lower mold parts 1, 2 and inputting an EVA blank 3 into the mold cavity 201, such as by overflowing or injecting; foaming and crosslinking the EVA blank 3 by heating and pressurizing; opening the mold and permitting the EVA blank 3 to expand continuously outside the mold to form a foam body 4; slicing and dividing the foam body 4 into a plurality of foam sheets 5; and piercing each of the foam sheets 5 through use of a solid pin 6 or a hollow pin 7 to form each foam sheet 5 with a plurality of ventilation holes 501.
Although the aforesaid method can provide the foam sheet 5 with the ventilation holes 501, it has the following shortcomings:
1. When the foam sheet 5 is pierced by the solid pin 6, the elasticity of portions of the foam sheet 5 near the ventilation holes 501 may be reduced due to the punching action of the solid pin 5.
2. Waste 502 of the foam sheet 5 as a result of removal by the hollow pin 7 can not be reused.
3. When the EVA blank 3 is foamed and crosslinked by heating, the portion of the EVA blank 3 distal from the mold has a slower reaction time as compared to the portion of the EVA blank 3 proximate to the mold. It is required to lengthen the reaction time so as to complete the foaming and crosslinking of the EVA blank 3. Therefore, a large amount of energy is required to complete the foaming of the EVA blank 3.
4. The ventilation holes 501 formed by the piercing of the solid pin 6 or the hollow pin 7 will result in scratches at the annular surfaces thereof due to the punching of the solid pin 6 or the hollow pin 7.